Descriptions

This investigation explored how the magnitude, style, and frequency of
channel adjustments vary spatially and over time within a 5th-order mountain
watershed. Historical data sets, including repeated cross section surveys spanning up
to 20 years at five sites on 2nd to 5th-order channels and streamflow records spanning
up to 50 years, were supplemented by mapping and field reconnaissance activities.
The study had two major parts. The first focused on two adjacent, contrasting
stream reaches to examine the influence of large woody debris (LWD) on channel
morphology and channel response to peak flows in a 3rd-order stream. The upper
reach flows through old-growth forest with abundant LWD, while the lower reach
was clearcut in 1964-65 and contains little LWD. A 25-year flood in 1996 caused
deposition upstream of LWD steps in the old-growth reach alternating with scour
between steps, resulting in no net gain or loss of sediment within the reach, while
extensive scour and coarsening of the bed occurred in the clearcut reach. These
observations suggest that reach-scale channel response was strongly influenced by
LWD abundance, but that response at finer scales depends critically on the details of
the location and arrangement of LWD.
The second part of the study examined the dynamics of channel response to
peak flows over two decades, and to two particular large floods during that period, in
different portions of the channel network. The cross section data show that the
streambed at the study sites is very stable, particularly in lower-order channels. Peak
flows that produced detectable change at 90% of cross sections-flows able to cause
significant channel adjustments-recur approximately three times as frequently
(every 6-7 years) in 4`" to 5`h-order Lookout Creek as in 3rd-order Mack Creek (20-25
years). Flows that produced detectable change at 25% of cross sections are estimated
to occur on average every 1.7 to 3.0 years at the study sites. It is estimated that if
peak flows of all sizes were increased by only 10% due to anthropogenic impacts
(e.g., logging) or climate change, the frequency of peak flows of a magnitude
observed to produce significant channel adjustments would increase by
approximately 30% in Lookout Creek and 60% in Mack Creek.